Significant oil and gas reserves have been discovered beneath various bodies of water throughout the world. Originally, the state of technology limited offshore drilling and production to relatively shallow locations in shoreline areas where the depth of the water ranged from a few feet to several hundred feet. The extensive exploration and removal of resources from these near shore regions, coupled with a constant demand for cost effective energy from large, productive reserves, have led to a search for and drilling of oil and gas reserves in locations beneath greater depths of water.
Presently, the industry is conducting drilling operations in depths of 9,000 feet of water, and it is anticipated that these operations will migrate to even deeper waters since the industry has begun leasing blocks for drilling in areas where the depth of water can be ten thousand feet or more. These desires will only grow as technology, such as seismic imaging, continues to progress and identify locations of substantial oil and gas reserves that are buried under even greater depths of water. The industry must still manage the shallower water oil and gas fields that have been or are continuing to be developed. When operating outside the major activity areas such as the Gulf of Mexico or the North Sea, limited drilling vessels are available in the locale so they must be mobilized from great distances around the globe at great cost for relatively short drilling programs.
In the past, shallow-water offshore drilling operations have been conducted from fixed towers and mobile units, such as jack-up platforms. These units are usually assembled on shore and then transported to an offshore drilling site. For a tower unit, the towers are erected over a proposed wellhead and fixed to the marine floor. A jack-up platform may be transported to the site through the use of a barge or through a self-propulsion mechanism on the platform itself. Once the platform is over the proper location, legs on the corners of the barge or a self-propelled deck are jacked down into the seabed until the deck is positioned above the statistical storm wave height. These jack-up barges and platforms drill through a relatively short conductor pipe usually thirty inch (30″) in diameter using a surface wellhead in a manner similar to land based operations. Although jack-up rigs and fixed platforms work well in depths of water that total approximately a few hundred feet, they do not work well in deep water operations. The Multi Purpose Unit (MPU) has been designed to work as a tender assisted drilling unit when operating alongside either a shallower water fixed platform or a floating deepwater production platform such as a Spar or TLP. In addition, the multi purpose unit has been designed to work as a stand alone mobile offshore drilling unit from water depths of a few hundred feet to many thousand feet using either a sub surface blow out preventer or a surface blow out preventer.
In a typical conventional offshore drilling operation a thirty inch (30″) casing is first jetted into the sea floor and is cemented into position to establish the well. Alternatively a thirty-six inch (36″) hole can be drilled and a thirty inch (30″) casing can be run and cemented. A twenty-six inch (26″) hole section is then drilled through the thirty inch (30″) casing. The twenty-six inch (26″) drilling assembly is then pulled back to the surface. Then a twenty inch (20″) tubular casing is run and landed on the wellhead housing that is attached to the top of the thirty inch (30″) casing. The twenty inch (20″) casing is then cemented into place. An eighteen and three-quarters inch (18¾″) blow out preventer (“BOP”) stack is connected to the bottom of a twenty-one inch (21″) riser and lowered onto the twenty inch (20″) high pressure wellhead housing that is attached to the top of the twenty inch (20″) casing. After this operation is completed and the twenty-one inch (21″) riser is set, all further drilling actually takes place through the single twenty-one inch (21″) riser. This includes drilling a seventeen and one-half inch (17½″) hole, running and cementing a thirteen and three-eighths inch (13⅜″) casing, drilling a twelve and one-quarter inch (12¼″) hole section, running and cementing a nine and five-eighths inch (9⅝″) casing, drilling an eight and one-half inch (8½″) hole, etc. Casing sizes and designs are program specific and therefore can be applied in many different combinations.
The present method is detailed below with reference to the listed Figures.